1. Field of the Invention
The present invention relates to a fan motor driving technique.
2. Description of the Related Art
In recent years, increase in the operation speed of personal computers and workstations has led to rapid increase in the operation speeds of computation LSIs (Large Scale Integrated Circuit) such as CPUs (Central Processing Unit), DSPs (Digital Signal Processor), etc. Such LSIs have a problem in that an increase in the operation speed, i.e., an increase in clock frequency involves an increase in heat generation. The heat generation of the LSI leads to thermal runaway of the LSI itself, or affects its peripheral circuits, which becomes a problem. Accordingly, such a situation requires a suitable thermal cooling operation for a heated component such as an LSI (which will be referred to as the “LSI” hereafter) as a crucial technique.
Examples of techniques for cooling an LSI includes an air-cooling cooling method employing a cooling fan. In this method, for example, a cooling fan is arranged such that it faces the surface of the LSI, and cool air is blown onto the surface of the LSI using the cooling fan.
The related conventional techniques are disclosed in Japanese Patent Application Laid Open No. H07-31190 and Japanese Patent Application Laid Open No. 2001-284868, for example.
FIG. 1 is a circuit diagram showing a configuration of a cooling apparatus investigated by the present inventors. A cooling apparatus 4 includes a driving apparatus 100 and a fan motor 6. The driving apparatus 100 is configured to drive the fan motor 6 according to a control signal S1 which indicates a torque (rotational speed) to be set for the fan motor 6.
The fan motor 6 is configured as a three-phase AC motor including a U-phase coil LU, a V-phase coil LV, and an W-phase coil LW, which are connected in a star winding, and an unshown permanent magnet. The driving apparatus 100 is configured as a function IC (Integrated Circuit) integrated on a single semiconductor substrate. The driving apparatus 100 is arranged such that a power supply voltage is supplied to its power supply terminal ICVDD, and a ground voltage is supplied to its ground terminal ICGND.
The driving apparatus 100 includes a back electromotive force (BEMF) detection circuit 10, a PWM input stage 12, a driving signal synthesizing circuit 14, a driving circuit 16, and a rotational speed signal generating circuit 20.
The PWM input stage 12 is configured to receive, as an input signal, the control signal S1 subjected to pulse width modulation according to the target torque of the fan motor 6, i.e., according to the target rotational speed thereof. When the maximum torque is indicated, the duty ratio of the control signal S1 becomes 100%. When the minimum torque (zero torque) is indicated, the duty ratio of the control signal S1 becomes 0%. The PWM input stage 12 is configured to perform duty ratio digital conversion so as to generate a torque instruction value S2 in the form of digital data that corresponds to the duty ratio of the control signal S1.
The BEMF detection circuit 10 is configured to compare each of the back electromotive force voltages VU, VV, and VW which develop at the respective terminals of the U-phase coil LU, V-phase coil LV, and W-phase coil LW, with an intermediate-point voltage VCOM that develops at a common connection node N1 that connects these three coils, and to generate a rotation detection signal S3 which is asserted for every electrical angle of 60 degrees. For example, the BEMF detection circuit 10 includes comparators (not shown) respectively provided to the U-phase coil, V-phase coil, and W-phase coil. Each comparator is configured to compare the coil voltage (back electromotive force voltage) VU, VV, or VW that occurs at one terminal of the corresponding phase coil with the intermediate-point voltage VCOM, and to generate a signal which indicates the comparison result. By logically combining the signals output from the respective phase comparators, such an arrangement generates the rotation detection signal S3.
The driving signal synthesizing circuit 14 is configured to receive the rotation detection signal S3 and the torque instruction value S2, and to combine the signals thus received so as to generate a driving control signal S4. Furthermore, immediately after the driving apparatus 100 is powered on, the driving signal synthesizing circuit 14 is configured to switch the driving sequence for the fan motor 6.
The driving circuit 16 is configured to apply a driving voltage to one terminal of each of the coils LU, LV, and LW, according to the driving control signal S4. The driving circuit 16 may be configured to BTL drive the fan motor 6, or may be configured to PWM drive the fan motor 6 according to the control signal S1.
The rotational speed signal generating circuit 20 is configured to generate a rotational speed signal FG that transits with every mechanical angle (motor angle) of 180 degrees with respect to the fan motor 6, i.e., every time the fan motor 6 rotates a half-turn, and to output the rotational speed signal FG thus generated via an FG terminal.
FIG. 2A is a circuit diagram showing an example configuration of the PWM input stage 12 investigated by the present inventors. FIG. 2B is a waveform diagram showing the operation thereof. The PWM input stage 12 shown in FIG. 2A includes a low-pass filter 12a and a smoothing circuit 12b. The low-pass filter 12a is configured as an IIR (Infinite Impulse Response) low-pass filter, for example. The smoothing circuit 12b is configured to perform sampling of a digital output value S10, which is an output signal of the low-pass filter 12a, with a sampling frequency that is the same as that of the control signal S1, so as to update the torque instruction value S2. It should be noted that the configuration and the operation of the PWM input stage 12 shown in FIG. 2 should by no means be recognized as known techniques.
With the PWM input stage 12 shown in FIG. 2A, when the duty ratio of the control signal S1 is maintained at a constant value, the torque instruction value S2 is also maintained at a constant value. Such an arrangement has an advantage of providing the electric motor with a stable torque. However, such an arrangement has a problem in that acoustic noise generated by the motor occurs at a particular frequency in a concentrated manner. There is a demand for providing a silent fan motor. Thus, such a problem must be solved.